The present invention is related to a test fixture for use in computerized automatic test equipment for checking printed circuit boards and, more particularly, to such a fixture which has been specially adapted to prevent damage from electrostatic charges to circuits and components mounted on it and to the tester circuitry.
Automatic test equipment for checking printed circuit boards has long been well known. For the sake of consistency and convenience, the printed circuit board and the circuitry thereon to be checked such as components and lead lines therebetween, will be referred to hereafter as a "unit under test", or "UUT". A standard and well known test approach utilizes a "bed of nails" test fixture in which a great number of arrayed, nail-like test probes is provided, each having a tip arranged to make electrical contact with a designated test point of the UUT. Any particular circuit laid out on a printed circuit board is likely to be different from all other circuits. Consequently, a "bed of nails" arrangement for contacting test points in a particular circuit must be customized for that circuit. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in a fixture. The typical fixture may have five-hundred test probes, and some even go up to two-thousand in number. An exemplary type of such test fixtures is available from Factron Schlumberger in Latham, N.Y. under the mark Thinline.
It is commonly known that electrostatic discharges can cause serious damage to semiconductor devices. Damage can occur by way of junction burnout when a localized voltage is induced by an electrostatic potential. If a sufficient current flows due to the induced electrostatic potential, thermal secondary breakdown can occur, thereby destroying the junction. Another type of damage is dielectric breakdown which occurs when the induced potentials are of a magnitude high enough to "punch through" the insulating layer of silicon dioxide which protects the gate of FETs and MOS-type semiconductors. Furthermore, metalization melt is another type of damage due to electrostatic discharge. Bond wires or metalization strips within the semiconductor burn out, like a fuse, when high currents induced by the electrostatic potential occur.
Various sources for developing an electrostatic potential exist in the work place. For example, electrostatic charge can accumulate due to the well known triboelectric effect generated during the normal act of walking across, for example, the surface of a rug. The worker accumulates a charge which creates a field between himself and nearby grounded objects. Any objects placed in that field will have a voltage induced on them depending on their size and orientation. Such a voltage can easily exceed five-thousand volts. Likewise, if a worker with such an accumulated static charge on him touches an electronic component, currents will flow through the semiconductor of the component and eventually to ground. The resultant voltage can cause dielectric breakdown or melt metalization.
It has been learned relatively recently that circuits are exposed to potentially damaging electrostatic charges in the course of being checked on automatic test equipment of the type such as briefly described above. An electrostatic charge can be built up on the printed circuit board as it is being moved from one location to another due to, for example, the triboelectric effect discussed above which occurs as the person carrying it walks along. Also, when vacuum is used to exert a force on the UUT to engage it against the bed of nails, air rushes through vents in the test fixture during the application of vacuum as well as the release of a vacuum. The friction of air particles moving against fixture surfaces is a source of electrostatic charge build-up. Furthermore, various fixture seals undergo a certain degree of motion as the vacuum is applied and the UUT is brought into engagement with the "bed of nails". This motion is a further source of electrostatic charge build-up.
Electrostatic voltages generated by the above-described causes can reach as high as one thousand volts from the top plate and up to five-thousand volts inside the vacuum well of the fixture. Such electrostatic potentials far exceed the static susceptibility range of MOS-FET chips (100 to 200 volts) as well as ECL bi-polar devices (up to 500 volts). A further concern besides rendering the semiconductor components inoperative is the electrostatic overstress effect. It can be caused even by an electrostatic charge which is only 25% of the static susceptibility values mentioned just above. This effect is such that the components seem to have survived the static without damage. However, it can cause parametric degradation, as well as cause the component to fail well before its specified mean time between failures. As integrated circuits are made with ever finer lines and ever thinner oxide layers, the danger of static will further aggravate the electrostatic overstress effect.
In order to protect the UUT from being damaged during the testing operation, it is desirable to accomplish several aims. Since it is not unlikely that an electrostatic charge will somehow accumulate on the test fixture, it is desirable to dissipate that charge relatively quickly, but not instantaneously such as undesirably occurs in an electostatic discharge to ground with its attendant damaging effects. It is also desirable to prevent the test fixture itself from generating an electrostatic charge during operation of the automatic test equipment. In addition, if an undissipated electrostatic charge remains on the test fixture, it is desirable to prevent a spark from passing from the UUT to the test fixture when, for example, the operator brings the UUT towards the test fixture for mounting thereon.
Various approaches have been developed in an attempt to effectively deal with the electrostatic charges developed on a test fixture due to a variety of causes including those mentioned above. One approach is to ground the operator, such as by a wrist strap connected to ground. Although this eliminates the build-up of electrostatic charges due to the effect of the worker, it completely fails to deal with the other causes listed above. Another approach utilizes an ionizer to blow ionized air across the fixture. However, its effect has been found to be limited because of the inability to access areas beneath the UUT where static charge is actually generated. In yet another approach, an antistatic spray is applied to the fixture. However, this requires disassembly of the fixture which is a somewhat laborious operation. Also, the spray has to be reapplied relatively frequently, such as at one-week intervals because it rubs off easily, evaporates (i.e. sublimes) or its electrical characteristics degrade. Also, such sprays are humidity dependent in their ability to prevent static buildup. Thus, each of the prior art approaches suffers from certain disadvantages which render it insufficiently effective or inconvenient and overly time consuming to use. Consequently, an improved approach is highly sought after.